Volume 91, No. 2 THE QUARTERLY REVIEW OF BIOLOGY June 2016 RESILIENCE TO DROUGHTS IN MAMMALS: A CONCEPTUAL FRAMEWORK FOR ESTIMATING VULNERABILITY OF A SINGLE SPECIES Tasmin L. Rymer College of Marine & Environmental Sciences and Centre for Tropical Manually chage ligatures Environmental & Sustainability Sciences, James Cook University Cairns, Queensland 4870 Australia School of Animal, Plant & Environmental Sciences, University of the Witwatersrand fi and fl to Johannesburg 2050 South Africa e-mail: [email protected] fi and fl Neville Pillay School of Animal, Plant & Environmental Sciences, University of the Witwatersrand Johannesburg 2050 South Africa e-mail: [email protected] Carsten Schradin School of Animal, Plant & Environmental Sciences, University of the Witwatersrand Johannesburg 2050 South Africa Institut Pluridisciplinaire Hubert Curien—Département Ecologie, Physiologie et Ethologie, Université de Strasbourg Strasbourg 67087 France CNRS, UMR7178 Strasbourg 67087 France e-mail: [email protected] keywords allostasis, homeostasis, metabolism, osmoregulation, preadaptation, thermoregulation The Quarterly Review of Biology, June 2016, Vol. 91, No. 2 Copyright © 2016 by The University of Chicago Press. All rights reserved. 0033-5770/2016/9102-0002$15.00 133 This content downloaded from 137.219.126.019 on May 31, 2016 18:08:34 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). 134 THE QUARTERLY REVIEW OF BIOLOGY Volume 91 abstract The frequency and severity of droughts in certain areas is increasing as a consequence of climate change. The associated environmental challenges, including high temperatures, low food, and water availability, have affected, and will affect, many populations. Our aims are to review the behavioral, physiological, and morphological adaptations of mammals to arid environments, and to aid research- ers and nature conservationists about which traits they should study to assess whether or not their study species will be able to cope with droughts. We provide a suite of traits that should be considered when making predictions about species resilience to drought. We define and differentiate between general adaptations, specialized adaptations, and exaptations, and argue that specialized adaptations are of little interest in establishing how nondesert specialists will cope with droughts. Attention should be placed on general adaptations of semidesert species and assess whether these exist as exaptations in nondesert species. We conclude that phenotypic flexibility is the most important general adaptation that may promote species resilience. Thus, to assess whether a species will be able to cope with increasing aridity, it is important to establish the degree of flexibility of traits identified in semidesert species that confer a fitness advantage under drying conditions. Introduction extinct, change their distribution range, or NTHROPOGENIC-induced environ- show an active response to change through Amental change, including habitat adaptive (Breed et al. 2011; Box 1) or exap- loss (Galbraith et al. 2002), fragmentation tive (Ghalambor et al. 2007; Box 1) phe- (Fahrig 2003), and climate change (Et- notypic plasticity. Interestingly, a number terson and Shaw 2001; Alley et al. 2003), of local declines and extinctions due to en- is accelerating at a rate unprecedented in vironmental change have been associated Earth’s history (Etterson and Shaw 2001; with changes in species interactions (Cahill Alley et al. 2003). In fact, the rate of ver- et al. 2013; Ockendon et al. 2014). How- tebrate species climatic niche evolution is ever, an individual’s ability to cope with its considerably slower than the current pre- prevailing environment is also dependent dicted rates of anthropogenic environmen- on its behavior, physiology, and morphol- tal change (Quintero and Wiens 2013). The ogy. These responses to abiotic factors then accelerated rates of human-induced envi- likely also influence species interactions, ronmental change could thus be driving a for example, through changes in activity sixth mass extinction event, requiring con- patterns, ultimately further impacting ex- servation efforts to be focused on reducing tinction risk. the rate of extinctions (Pimm 2009; Black One of the main consequences of global et al. 2011; Harley 2011). However, we are climate change is an increase in the frequency currently unable to predict which species and intensity of droughts (IPCC 2007; Dai will be threatened by extinction under in- 2011). Regions where droughts have be- creased environmental change and which come, and are projected to become, longer species will survive (Moritz and Agudo and more frequent include the Mediterra- 2013). nean, central Europe, central North America, Many studies have assessed the proba- and southern Africa (IPCC 2007). Although bility and potential of species to adapt to the latest IPCC report (2013) mentions a rapid environmental change via evolution- lower confidence for the increased incidence ary adaptation (e.g., evolutionary rescue; of droughts since the 1950s, it is still pre- Gonzalez et al. 2013; Box 1). However, evo- dicted that droughts are likely to increase lutionary adaptation is generally a slow in some areas in the future. Droughts are pro cess, occurring over generations (Hoff- a normal part of climate variation, which mann and Sgrò 2011), and may occur too means that, independent of global climate slowly for species to respond to rapid envi- change, natural populations have and ronmental and climate change. Under will continue to face this problem repeat- such circumstances, species may become edly. For example, in the last 500 years, This content downloaded from 137.219.126.019 on May 31, 2016 18:08:34 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). June 2016 SPECIES RESILIENCE TO DROUGHTS 135 BOX 1 Definition of terms Term Definition Adaptive phenotypic plasticity Alteration of some aspect of the phenotype in re- sponse to prevailing environmental conditions, which increases an individual’s chance of survival and repro- duction (maximizes fitness; Charmantier et al. 2008; Ozgul et al. 2009, 2010; Piersma and van Gils 2010) and that may facilitate population persistence and potentially evolutionary adaptation (Hoffmann and Sgrò 2011). Traits that specifically evolved in response to historical selection pressures and have current util- ity in the same context (phenotypic adaptation). (100S − 60D) Aridity index Im = ; where Im = index of moisture PEt availability; S = total moisture surplus (mm) from all months with a water surplus; D = total of all monthly deficiencies (mm); and PE = annual po- tential evapotranspiration (mm; Meigs 1953). Both S and D are represented by the difference between monthly precipitation and monthly potential evapo- transpiration (Nash and Engineering Group Work- ing Party 2012). Deserts have an aridity index below 0.2, semideserts between 0.2 and 0.5. Behavioral evader Animals able to escape environmental extremes us- ing primarily behavioral and physiological adapta- tions (Willmer et al. 2000). Developmental plasticity Nonreversible phenotypic plasticity occurring during ontogeny (Piersma and Lindström 1997). Drought Current precipitation falls significantly below the long-term mean of a defined area (Dai 2011). Is defined on a temporal scale and can occur in any environment. Drought-affected areas Areas that are neither deserts nor semideserts, but ex- perience unpredicted droughts, which might occur more often and become more intense in the future due to climate change (Li et al. 2009). In the fu- ture, increasingly more areas might become drought prone due to climate change. Environmental stressor Any stimulus that could affect an individual nega- tively if it cannot respond to it via a specific stress response, which itself is costly (e.g., Charmandari et al. 2005). Typically uncontrollable for the individ- ual, but can be predictable (e.g., seasonal changes in food) or unpredictable (e.g., a storm; Wingfield 2013). continued This content downloaded from 137.219.126.019 on May 31, 2016 18:08:34 PM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c). 136 THE QUARTERLY REVIEW OF BIOLOGY Volume 91 BOX 1 Continued Term Definition Evolutionary adaptation Change in the relative gene frequencies of alleles (and associated phenotypic changes) within a popu- lation over multiple generations, with a correspond- ing increase in species fitness over time (Pianka 2011; Rezende and Diniz-Filho 2012). Exaptation Formerly called preexisting adaptations or preadap- tations. Traits that evolved in response to historical selection pressures, which have current utility but in a different context (i.e., different environment or selection pressures; Gould and Lewontin 1979). Exaptive phenotypic plasticity Alteration of traits in response to prevailing envi- ronmental conditions, which specifically evolved in response to historical selection pressures and have current utility in a different context (exaptation), increasing an individual’s chance of survival and reproduction (increasing fitness) in novel environ- ments (Ghalambor et al. 2007). Interannual rainfall variability Expressed as a percentage of the mean deviation of precipitation from the average over the average pre- cipitation (Nash and Engineering Group Working Party 2012). Phenotypic flexibility Reversible